The invention relates to a receiver-transmitter for a target identification system having a particular but not necessarily an exclusive application for the identification of aircraft.
The identification of targets, such as aircraft, utilising lasers involves the coherent detection of scattered light from the targets which in the case of aircraft, would be travelling at high speed and would, therefore, produce a large Doppler-shift in the received signal. For a laser wavelength of 1 .mu.m, the Doppler shift is 2 MHz per meter/second of relative target velocity. Thus, for aircraft travelling towards an observer at Mach 2, the Doppler shift is of the order of 1 GHz. A laser vibrometer detection system would have to lock onto this Doppler frequency and measure the frequency modulation imposed by the target vibration.
With such an arrangement, the FM sidebands would be only a few kHz away from the centre frequency and so could be difficult to detect, especially if the signal-to-noise ratio of the return signal from the target is low.
A major problem with such systems is laser frequency stability which introduces FM noise into the coherently detected signal, and so affects the ability of the system to detect signal sidebands close-in to the carrier. This is a serious problem at long range, where the long time delay between transmitted and return signals introduces a large amount of phase-decorrelation between signal and reference beams in the receiver, leading to increased FM noise.
The identification of targets utilising lasers and, in particular, laser vibrometry is normally effected by focusing a laser beam onto a diffraction-limited spot on the surface of the target and by detecting the backscatter from the target by means of a telescope located adjacent to or coaxial with the laser transmitter. The light wave received at the telescope is focused onto a photodetector where it is combined with a low-power reference wave derived from the laser transmitter. Interference between the two waves generates a heterodyne photocurrent on the photodetector which is selected by a phase-locked loop tracking filter. Periodic variations in the Doppler frequency due to target vibration appear as a modulation voltage in the phase-locked loop output.
A system for detecting vibration signatures from targets, such as aircraft, will require either some means for locking the transmitted beam onto the relevant part of the aircraft surface whilst the vibration signature is being detected, or a relatively wide-field transmitted beam which illuminates a substantial area and does not require precise beam positioning on the target.
In view of the difficulty of locking beams onto high speed targets, such as aircraft, some form of array receiver is required for detecting/identifying such targets.